db_lock



NAME

       db_lock - lock manager


SYNOPSIS

       #include <db.h>

       int
       lock_open(const char *dir,
            int flags, int mode, DB_ENV *dbenv, DB_LOCKTAB **regionp);

       int
       lock_id(DB_LOCKTAB *lt, u_int32_t *idp);

       int
       lock_vec(DB_LOCKTAB *lt, u_int32_t locker, int flags,
            DB_LOCKREQ list[], int nlist, DB_LOCKREQ **elistp);

       int
       lock_get(DB_LOCKTAB *lt, u_int32_t locker, int flags,
            const DBT *obj, const db_lockmode_t lock_mode, DB_LOCK *lock);

       int
       lock_put(DB_LOCKTAB *lt, DB_LOCK lock);

       int
       lock_close(DB_LOCKTAB *lt);

       int
       lock_unlink(const char *dir, int force, DB_ENV *dbenv);

       int
       lock_detect(DB_LOCKTAB *lt, db_detect_t atype, int flags);


DESCRIPTION

       The  DB  library  is  a family of groups of functions that
       provides a modular programming interface  to  transactions
       and  record-oriented  file  access.   The library includes
       support for transactions, locking, logging and  file  page
       caching,  as well as various indexed access methods.  Many
       of the functional groups  (e.g.,  the  file  page  caching
       functions)  are  useful  independent of the other DB func-
       tions, although  some  functional  groups  are  explicitly
       based  on  other functional groups (e.g., transactions and
       logging).  For a general description of  the  DB  package,
       see db_intro(3).

       This  manual  page  describes  the specific details of the
       locking interface.

       The db_lock functions are the library  interface  intended
       to  provide  general-purpose  locking.   While designed to
       work with the other DB functions, these functions are also
       useful  for  more  general locking purposes.  Locks can be
       shared between processes.  In most  cases,  when  multiple
       Otherwise, if dir is not NULL, it is interpreted  relative
       to  the  current working directory of the process.  If dir
       is NULL, the following environment variables  are  checked
       in  order:  ``TMPDIR'',  ``TEMP'', and ``TMP''.  If one of
       them is set, lock table files are created relative to  the
       directory  it  specifies.   If  none  of them are set, the
       first possible one of the following directories  is  used:
       /var/tmp, /usr/tmp, /temp, /tmp, C:/temp and C:/tmp.

       All  files  associated  with the lock table are created in
       this directory.  This directory must already exist when is
       called.   If  the  lock  table already exists, the process
       must have permission to read and write the existing files.
       If the lock table does not already exist, it is optionally
       created and initialized.

       The flags and mode arguments specify  how  files  will  be
       opened  and/or created when they don't already exist.  The
       flags value is specified by or'ing together one or more of
       the following values:

       DB_CREATE
            Create  any  underlying  files, as necessary.  If the
            files do not already exist and the DB_CREATE flag  is
            not specified, the call will fail.

       DB_THREAD
            Cause the DB_LOCKTAB handle returned by the lock_open
            function to be useable by multiple threads  within  a
            single  address space, i.e., to be ``free-threaded''.

       All files created by the lock subsystem are  created  with
       mode  mode  (as described in chmod(2)) and modified by the
       process'  umask  value  at  the  time  of  creation   (see
       umask(2)).   The group ownership of created files is based
       on the system and directory defaults, and is  not  further
       specified by DB.

       The  locking  subsystem  is  configured based on the dbenv
       argument to lock_open, which is a pointer to  a  structure
       of type DB_ENV (typedef'd in <db.h>).  It is expected that
       applications will use a single  DB_ENV  structure  as  the
       argument  to  all of the subsystems in the DB package.  In
       order to ensure compatibility with future releases of  DB,
       all fields of the DB_ENV structure that are not explicitly
       set should be initialized to 0 before the first  time  the
       structure  is  used.   Do  this by declaring the structure
       external or static, or by calling the  C  library  routine
       bzero(3) or memset(3).

       The  fields  of the DB_ENV structure used by lock_open are
       described below.  As references to  the  DB_ENV  structure
       may  be  maintained by lock_open, it is necessary that the
       DB_ENV structure and memory it references be  valid  until
            The error fields of the DB_ENV  behave  as  described
            for db_appinit(3).

       const u_int8_t lk_conflicts[][];
            A  lk_modes by lk_modes array.  A non-0 value for the
            array element:

                 lk_conflicts[requested_mode][held_mode]

            indicates that requested_mode and held_mode conflict.
            The  ``not-granted''  mode  must be represented by 0.
            If  lk_conflicts  is  NULL,   the   conflicts   array
            db_rw_conflicts  is used; see the section below enti-
            tled ``STANDARD LOCK MODES''  for  a  description  of
            that array.

       db_detect_t lk_detect;
            If non-0, specifies that the deadlock detector be run
            whenever a lock conflict occurs, and specifies  which
            transaction  should be aborted in the case of a dead-
            lock.  The lk_detect field must be set to one of  the
            following values.

            DB_LOCK_DEFAULT
                 Use  the  default  policy  as  specified  in the
                 db_deadlock(1) man page.

            DB_LOCK_OLDEST
                 Abort the oldest transaction.

            DB_LOCK_RANDOM
                 Abort a random transaction involved in the dead-
                 lock.

            DB_LOCK_YOUNGEST
                 Abort the youngest transaction.

       unsigned int lk_max;
            The  maximum  number of locks to be held or requested
            in the table.  This value is  used  by  lock_open  to
            estimate how much space to allocate for various lock-
            table data structures.  If lk_max  is  0,  a  default
            value is used.

       int lk_modes;
            The number of lock modes to be recognized by the lock
            table  (including  the  ``not-granted''  mode).    If
            lk_modes  is  0,  the value DB_LOCK_RW_N is used; see
            below for a description of that value.

       The lock_open function returns the value of errno on fail-
       ure and 0 on success.

  lock_id
       and 0 on success.

  lock_vec
       The  lock_vec function atomically obtains and releases one
       or more locks from the specified table, lt.  The  lock_vec
       function  is intended to support acquisition or trading of
       multiple locks under  one  lock  table  semaphore,  as  is
       needed  for  lock  coupling or in multigranularity locking
       for lock escalation.

       The locker argument specified to lock_vec is  an  unsigned
       32-bit   integer   quantity.   It  represents  the  entity
       requesting or releasing the lock.

       The flags value must be set to 0 or the following value:

       DB_LOCK_NOWAIT
            If a lock cannot be  granted  because  the  requested
            lock  conflicts with an existing lock, return immedi-
            ately instead of  waiting  for  the  lock  to  become
            available.

       The list array provided to lock_vec is typedef'd in <db.h>
       as DB_LOCKREQ.  A DB_LOCKREQ structure has  at  least  the
       following fields, which must be initialized before calling
       lock_vec:

       lockop_t op;
            The operation to be performed, which must be  set  to
            one of the following values:

            DB_LOCK_GET
                 Get  a lock, as defined by the values of locker,
                 obj and mode.  Upon return from lock_vec, if the
                 lock  field  is  non-NULL,  a  reference  to the
                 acquired lock is stored there.  (This  reference
                 is  invalidated  by  any  call  to  lock_vec  or
                 lock_put that releases the lock.)

            DB_LOCK_PUT
                 The lock referenced by the contents of the  lock
                 field is released.

            DB_LOCK_PUT_ALL
                 All locks held by the locker are released.  (Any
                 locks acquired as a part of the current call  to
                 lock_vec  that  appear after the DB_LOCK_PUT_ALL
                 entry are not considered for this operation).

            DB_LOCK_PUT_OBJ
                 All locks held by the locker, on the object obj,
                 with   the  mode  specified  by  lock_mode,  are
                 released.  A lock_mode of  DB_LOCK_NG  indicates
                 that all locks on the object should be released.

       DB_LOCK lock;
            A lock reference.

       The nlist argument specifies the number of elements in the
       list array.

       If any of the requested locks cannot be acquired,  or  any
       of the locks to be released cannot be released, the opera-
       tions before the failing operation are guaranteed to  have
       completed  successfully,  and  lock_vec returns a non-zero
       value.  In addition, if elistp is not NULL, it is  set  to
       point  to  the  DB_LOCKREQ  entry that was being processed
       when the error occurred.

       In the case of an error, lock_vec may return  one  of  the
       following values:

       DB_LOCK_DEADLOCK
            The  specified  locker  was  selected  as a victim in
            order to resolve a deadlock.

       DB_LOCK_NOTHELD
            The lock cannot be released, as it was  not  held  by
            the locker.

       DB_LOCK_NOTGRANTED
            A  lock  was  requested that could not be granted and
            the flag parameter was  set  to  DB_LOCK_NOWAIT.   In
            this case, if non-NULL, elistp identifies the request
            that was granted.

       Otherwise, the lock_vec  function  returns  the  value  of
       errno on failure and 0 on success.

  lock_get
       The  lock_get  function  is  a  simple  interface  to  the
       lock_vec functionality, and is equivalent to  calling  the
       lock_vec function with the lt and locker arguments, elistp
       and conflict arguments, and a single element  list  array,
       for  which  the  op  field  is  DB_LOCK_GET,  and the obj,
       lock_mode and lock fields are represented by the arguments
       of  the same name.  Note that the type of the obj argument
       to lock_get is different from the obj element found in the
       DB_LOCKREQ  structure.  The lock_get function returns suc-
       cess and failure as described for the lock_vec function.

  lock_put
       The  lock_put  function  is  a  simple  interface  to  the
       lock_vec  functionality,  and is equivalent to calling the
       lock_vec function with a single element  list  array,  for
       which  the  op  field is DB_LOCK_PUT and the lock field is
       represented  by  the  argument  of  the  same  name.   The
       lock_put function returns success and failure as described
       dbenv was not initialized using db_appinit, all files cre-
       ated  for  this  shared  region  will  be  removed,  as if
       lock_unlink were called.

       When multiple threads are using the DB_LOCKTAB handle con-
       currently,  only  a  single thread may call the lock_close
       function.

       The lock_close function returns  the  value  of  errno  on
       failure and 0 on success.


  lock_unlink
       The  lock_unlink  function destroys the lock table identi-
       fied by the directory dir,  removing  all  files  used  to
       implement  the  lock  table.   (The  directory  dir is not
       removed.)   If  there  are  processes  that  have   called
       lock_open without calling lock_close (i.e., there are pro-
       cesses currently using the lock table),  lock_unlink  will
       fail without further action, unless the force flag is set,
       in which case lock_unlink will attempt to remove the  lock
       table  files  regardless  of any processes still using the
       lock table.

       The result of attempting to forcibly  destroy  the  region
       when  a  process has the region open is unspecified.  Pro-
       cesses using a shared memory region maintain an open  file
       descriptor  for  it.   On UNIX systems, the region removal
       should succeed and processes that have already joined  the
       region  should  continue  to  run  in  the  region without
       change, however processes  attempting  to  join  the  lock
       table  will either fail or attempt to create a new region.
       On other systems, e.g., WNT, where  the  unlink(2)  system
       call  will fail if any process has an open file descriptor
       for the file, the region removal will fail.

       In the case of catastrophic or  system  failure,  database
       recovery  must  be  performed  (see  db_recovery(1) or the
       DB_RECOVER flags  to  db_appinit(3)).   Alternatively,  if
       recovery  is  not  required  because  no database state is
       maintained across failures, it is possible to clean  up  a
       lock  table  by removing all of the files in the directory
       specified to the lock_open function, as lock  table  files
       are  never  created  in  any  directory other than the one
       specified to lock_open.  Note, however, that this has  the
       potential  to remove files created by the other DB subsys-
       tems in this database environment.

       The lock_unlink function returns the  value  of  errno  on
       failure and 0 on success.
  lock_detect
       The  lock_detect  function runs one iteration of the dead-
       lock detector on the specified table,  lt.   The  deadlock
       detector  traverses the lock table, detects deadlocks, and
       DB_LOCK_CONFLICT
            Only run the deadlock detector if a lock conflict has
            occurred since the last time that the deadlock detec-
            tor was run.

       The  lock_detect  function  returns  the value of errno on
       failure and 0 on success.

       The lock_detect function is the underlying  function  used
       by  the  db_deadlock(1)  utility.  See the source code for
       the  db_deadlock  utility  for   an   example   of   using
       lock_detect in a UNIX environment.


ENVIRONMENT VARIABLES

       The  following  environment variables affect the execution
       of db_lock:

       DB_HOME
            If the dbenv argument to  lock_open  was  initialized
            using  db_appinit,  the  environment variable DB_HOME
            may be used as the path of the database home for  the
            interpretation  of  the dir argument to lock_open, as
            described in db_appinit(3).

       TMPDIR
            If the dbenv argument to lock_open was  NULL  or  not
            initialized  using  db_appinit, the environment vari-
            able TMPDIR may be used as the directory in which  to
            create  the lock table, as described in the lock_open
            section above.


STANDARD LOCK MODES

       The include file <db.h> declares two  commonly  used  con-
       flict arrays:

       const u_int8_t db_lock_rw_conflicts[];
            This  is  a  conflict array for a simple scheme using
            shared and exclusive lock modes.
       const u_int8_t db_lock_riw_conflicts[];
            This is a conflict array that involves various intent
            lock  modes  (e.g.,  intent shared) that are used for
            multigranularity locking.

       Their associated sizes are DB_LOCK_RW_N and DB_LOCK_RIW_N.

       In  addition,  the  include  file  <db.h> defines the type
       db_lockmode_t, which is the type of the  lock  modes  used
       with the standard tables above:

              DB_LOCK_NG
                   not granted (always 0)

              DB_LOCK_READ
                   read (shared)

       In  addition,  the  lock_open function may fail and return
       errno for the following conditions:

       [EAGAIN]
            The shared memory region was locked and  (repeatedly)
            unavailable.

       [EINVAL]
            An invalid flag value or parameter was specified.

            The  DB_THREAD  flag  was specified and spinlocks are
            not implemented for this architecture.

       The lock_vec function may fail and return errno for any of
       the  errors  specified  for  the  following DB and library
       functions: fcntl(2), fflush(3), lock_detect(3), lseek(2),
       memcpy(3), memset(3), mmap(2), munmap(2), strerror(3), and
       write(2).

       In addition, the lock_vec function may fail and return er-
       rno for the following conditions:

       [EACCES]
            An  attempt  was made to release lock held by another
            locker.
       [EINVAL]
            An invalid flag value or parameter was specified.

       The lock_get function may fail and return errno for any of
       the  errors  specified  for  the  following DB and library
       functions: fcntl(2), fflush(3), lock_detect(3), lseek(2),
       memcpy(3), memset(3), mmap(2), munmap(2), strerror(3), and
       write(2).

       In addition, the lock_get function may fail and return er-
       rno for the following conditions:

       [EINVAL]
            An invalid flag value or parameter was specified.

       The lock_put function may fail and return errno for any of
       the errors specified for  the  following  DB  and  library
       functions: fcntl(2), fflush(3), lock_detect(3), lseek(2),
       memcpy(3), memset(3), mmap(2), munmap(2), strerror(3), and
       write(2).

       In addition, the lock_put function may fail and return er-
       rno for the following conditions:

       [EACCES]
            An attempt was made to release lock held  by  another
            locker.

       sigfillset(3), sigprocmask(2), stat(2), strcpy(3),
       strdup(3), strerror(3), strlen(3), and unlink(2).

       In  addition, the lock_unlink function may fail and return
       errno for the following conditions:

       [EBUSY]
            The shared memory region was in  use  and  the  force
            flag was not set.

       The lock_detect function may fail and return errno for any
       of the errors specified for the following DB  and  library
       functions: calloc(3), fcntl(2), fflush(3), lseek(2),
       malloc(3), memcpy(3), memset(3), mmap(2), munmap(2),
       strerror(3), and write(2).


BUGS

       If  a process dies while holding locks, those locks remain
       held and are never released.  In this case, all  processes
       should exit as quickly as possible, so that db_recover can
       be run.


SEE ALSO

       db_archive(1), db_checkpoint(1), db_deadlock(1), db_dump(1),
       db_load(1), db_recover(1), db_stat(1), db_intro(3),
       db_appinit(3), db_cursor(3), db_dbm(3), db_jump(3), db_lock(3),
       db_log(3), db_mpool(3), db_open(3), db_thread(3), db_txn(3)